Wavelength selective switches have one or more input ports and are capable of routing each of a number of wavelengths on the input ports to an output port selected for that wavelength. Reconfigurable add/drop multiplexers have one or more input ports, one or more output ports and can add and/or drop wavelengths before producing signals at the output ports.
In some cases, an attenuation function is implemented on one or more wavelengths.
In general, it is useful to have optical performance monitoring at the output of such devices to verify switching precision and/or attenuation accuracy, and in some cases to allow feedback functions to be performed. The conventional way to perform optical performance monitoring is to put a monitor on each output port so as to allow an immediate determination of the transfer function between the input and that output port. For a large number of output ports, in order to monitor all output ports, multiple instances of the optical performance monitoring equipment is required, or the equipment needs to be disconnected from each port and connected to each other port in sequence for example with an optical switch. These two approaches are either very expensive or time consuming and tedious. Also, if closed loop feedback is to be performed, there would be the necessity for a respective optical performance monitor per port.